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Fig. 6. Axon navigation in the retina under NrCAM inhibition. (A-D) Axons growing
towards the optic fissure in a retina flat-mount under NrCAM inhibition (top
view). (A) Four axons (marked 1-4) grow on pre-existing axons and have large,
complex growth cones. Axon 1 forms a growth cone with wide, laterally
exploring protrusions (B), detaches from the other axons and turns away at an
almost rectangular angle (C). This aberrant growth direction, perpendicular to
the pathway to the optic fissure, is maintained for more than 20 minutes (D).
Axon 2 turns away from the other axons at a smoother angle (B,C); later it
performs a compensatory turn, returning to the correct direction (D). At 54
minutes, axon 3 also starts to deflect (D). Axon 4 grows towards the optic
fissure during the observation period, displaying a growth behaviour typically
found under control conditions. (E,F) Growth cone kinetic plots of two axons
each, growing in retina flat-mounts in control conditions (E) and with NrCAM
inhibition (F); each dot represents the position of the growth cone neck
localised every two minutes (observation time in brackets). The lower plot in
F corresponds to axon 2 shown in A-D. Under control conditions, axons grow
rather straight and steadily towards the optic fissure (E). Under NrCAM
inhibition the pathway is more crooked and deviating from the correct
direction; in addition long pauses and retractions are observed (F). The
overall distance covered is considerably shorter under NrCAM inhibition; note
that the observation period is about 20% longer in F than in E. Scale bar: 20
µm.
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